‘C’olony forming units
EU Directive 98/83 (amended in 2015) states that the European standard for drinking water should be fewer than 100 CFUs in the potable supply 2. For water used in DUWLs and autoclaves, the current recommendation for England, Wales and Northern Ireland is 100 to 200 CFU/ml (as detailed in section 6.79 of HTM 01-05) 3, even after any POU filtration is used.
However, extraneous factors can suddenly push this figure up in some clinical situations, often as a result of internal conditions being ideal breeding grounds for pathogens: warmth, moisture, extended periods of inactivity (stagnation), dead-legs in plumbing layout and processed water that has had the chlorine removed. Research has revealed that bacterial counts in DUWLs can be enormous, with one study on www.cqc.org.uk demonstrating an incredible colony size of 19,500 CFUs 4. It does not necessarily follow that exposure to pathogens will lead to disease; rather, it depends on the virulence and dose of microbes, and the robustness of the host’s immune system 5. However, those who are immunocompromised – for example, the elderly and those with HIV, smokers, diabetics, and alcoholics – may become infected by the likes of Legionella and Pseudomonas Aeruginosa 5,6 .
This is not conjecture; a California-based dentist became seriously ill after contracting Legionnaires’ disease, while, more recently, an Italian elderly woman died as a result of contamination via DUWLs at her dental practice 6.
‘D’iscussing water sources
Purified water is integral to the smooth running of two key pieces of dental equipment – namely, dental chairs and autoclaves.
This begs the question – from where should water for DUWLs (chairs) be sourced and how can this be achieved? The answer depends on your dental chair design, with two types available. ‘Mains fed’ chairs require a permanent connection to potable water and thus have a constant supply feeding the waterlines. More commonly in the UK, ‘self-contained’ dental chair systems have independent water reservoirs and require reverse osmosis (RO) or distilled water, to adhere to HTM01/05 ‘best practice’ advice (sec. 6.84) 3.
Cross-infection specialist Caroline Pankhurst et al (2017) has this to say on the subject: ‘Although water can be taken from the mains supply (potable water), to achieve the specified water quality, dental units are usually fitted with a separate water reservoir that is independent of the public water supply. This allows dentists to have better control over the microbial quality of the water used in patient care by the addition of biocides and other methods to control contamination. In addition, they act as a type A air-gap, a physical gap that prevents back siphonage of contaminated water into the mains supply. Reservoir water bottles are recommended to be filled with freshly produced (less than 12 hours old) reverse osmosis or distilled water. These purified waters are not sterile but are unlikely to contain NTMs [Nontuberculous Mycobacteria] and pseudomonads found in potable tap water’ 6.
Furthermore, technical guidance HSG274 (part 3) of the L8 Approved Code of Practice recommends that storage bottles should be cleaned, rinsed with RO or distilled water, drained and inverted overnight’ 7 .
As for autoclaves (steam sterilisers), section 4.11 of HTM 01-05 states: ‘Each sterilizer …should be filled, at least daily, using distilled or RO water’ 3 .